Commit 23a80d46 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

Merge tag 'rcu.release.v6.8' of https://github.com/neeraju/linux 

Pull RCU updates from Neeraj Upadhyay:

 - Documentation and comment updates

 - RCU torture, locktorture updates that include cleanups; nolibc init
   build support for mips, ppc and rv64; testing of mid stall duration
   scenario and fixing fqs task creation conditions

 - Misc fixes, most notably restricting usage of RCU CPU stall
   notifiers, to confine their usage primarily to debug kernels

 - RCU tasks minor fixes

 - lockdep annotation fix for NMI-safe accesses, callback
   advancing/acceleration cleanup and documentation improvements

* tag 'rcu.release.v6.8' of https://github.com/neeraju/linux:
  rcu: Force quiescent states only for ongoing grace period
  doc: Clarify historical disclaimers in memory-barriers.txt
  doc: Mention address and data dependencies in rcu_dereference.rst
  doc: Clarify RCU Tasks reader/updater checklist
  rculist.h: docs: Fix wrong function summary
  Documentation: RCU: Remove repeated word in comments
  srcu: Use try-lock lockdep annotation for NMI-safe access.
  srcu: Explain why callbacks invocations can't run concurrently
  srcu: No need to advance/accelerate if no callback enqueued
  srcu: Remove superfluous callbacks advancing from srcu_gp_start()
  rcu: Remove unused macros from rcupdate.h
  rcu: Restrict access to RCU CPU stall notifiers
  rcu-tasks: Mark RCU Tasks accesses to current->rcu_tasks_idle_cpu
  rcutorture: Add fqs_holdoff check before fqs_task is created
  rcutorture: Add mid-sized stall to TREE07
  rcutorture: add nolibc init support for mips, ppc and rv64
  locktorture: Increase Hamming distance between call_rcu_chain and rcu_call_chains
parents 38814330 7dfb03dd

Documentation/RCU/checklist.rst

+16 −9
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@@ -241,15 +241,22 @@ over a rather long period of time, but improvements are always welcome! 
	srcu_struct.  The rules for the expedited RCU grace-period-wait

	primitives are the same as for their non-expedited counterparts.



	If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(),

	then the readers must refrain from executing voluntary

	context switches, that is, from blocking.  If the updater uses

	call_rcu_tasks_trace() or synchronize_rcu_tasks_trace(), then

	the corresponding readers must use rcu_read_lock_trace() and

	rcu_read_unlock_trace().  If an updater uses call_rcu_tasks_rude()

	or synchronize_rcu_tasks_rude(), then the corresponding readers

	must use anything that disables preemption, for example,

	preempt_disable() and preempt_enable().

	Similarly, it is necessary to correctly use the RCU Tasks flavors:



	a.	If the updater uses synchronize_rcu_tasks() or

		call_rcu_tasks(), then the readers must refrain from

		executing voluntary context switches, that is, from

		blocking.



	b.	If the updater uses call_rcu_tasks_trace()

		or synchronize_rcu_tasks_trace(), then the

		corresponding readers must use rcu_read_lock_trace()

		and rcu_read_unlock_trace().



	c.	If an updater uses call_rcu_tasks_rude() or

		synchronize_rcu_tasks_rude(), then the corresponding

		readers must use anything that disables preemption,

		for example, preempt_disable() and preempt_enable().



	Mixing things up will result in confusion and broken kernels, and

	has even resulted in an exploitable security issue.  Therefore,

Documentation/RCU/rcu_dereference.rst

+20 −7
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@@ -3,13 +3,26 @@ 
PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference()

===============================================================



Most of the time, you can use values from rcu_dereference() or one of

the similar primitives without worries.  Dereferencing (prefix "*"),

field selection ("->"), assignment ("="), address-of ("&"), addition and

subtraction of constants, and casts all work quite naturally and safely.



It is nevertheless possible to get into trouble with other operations.

Follow these rules to keep your RCU code working properly:

Proper care and feeding of address and data dependencies is critically

important to correct use of things like RCU.  To this end, the pointers

returned from the rcu_dereference() family of primitives carry address and

data dependencies.  These dependencies extend from the rcu_dereference()

macro's load of the pointer to the later use of that pointer to compute

either the address of a later memory access (representing an address

dependency) or the value written by a later memory access (representing

a data dependency).



Most of the time, these dependencies are preserved, permitting you to

freely use values from rcu_dereference().  For example, dereferencing

(prefix "*"), field selection ("->"), assignment ("="), address-of

("&"), casts, and addition or subtraction of constants all work quite

naturally and safely.  However, because current compilers do not take

either address or data dependencies into account it is still possible

to get into trouble.



Follow these rules to preserve the address and data dependencies emanating

from your calls to rcu_dereference() and friends, thus keeping your RCU

readers working properly:



-	You must use one of the rcu_dereference() family of primitives

	to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU

Documentation/RCU/torture.rst

+1 −1
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@@ -185,7 +185,7 @@ argument. 
Not all changes require that all scenarios be run.  For example, a change

to Tree SRCU might run only the SRCU-N and SRCU-P scenarios using the

--configs argument to kvm.sh as follows:  "--configs 'SRCU-N SRCU-P'".

Large systems can run multiple copies of of the full set of scenarios,

Large systems can run multiple copies of the full set of scenarios,

for example, a system with 448 hardware threads can run five instances

of the full set concurrently.  To make this happen::

Documentation/admin-guide/kernel-parameters.txt

+6 −0
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@@ -5313,6 +5313,12 @@ 
			Dump ftrace buffer after reporting RCU CPU

			stall warning.



	rcupdate.rcu_cpu_stall_notifiers= [KNL]

			Provide RCU CPU stall notifiers, but see the

			warnings in the RCU_CPU_STALL_NOTIFIER Kconfig

			option's help text.  TL;DR:  You almost certainly

			do not want rcupdate.rcu_cpu_stall_notifiers.



	rcupdate.rcu_cpu_stall_suppress= [KNL]

			Suppress RCU CPU stall warning messages.

Documentation/memory-barriers.txt

+10 −7
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@@ -396,10 +396,11 @@ Memory barriers come in four basic varieties: 




 (2) Address-dependency barriers (historical).

     [!] This section is marked as HISTORICAL: For more up-to-date

     information, including how compiler transformations related to pointer

     comparisons can sometimes cause problems, see

     Documentation/RCU/rcu_dereference.rst.

     [!] This section is marked as HISTORICAL: it covers the long-obsolete

     smp_read_barrier_depends() macro, the semantics of which are now

     implicit in all marked accesses.  For more up-to-date information,

     including how compiler transformations can sometimes break address

     dependencies, see Documentation/RCU/rcu_dereference.rst.



     An address-dependency barrier is a weaker form of read barrier.  In the

     case where two loads are performed such that the second depends on the
@@ -560,9 +561,11 @@ There are certain things that the Linux kernel memory barriers do not guarantee: 


ADDRESS-DEPENDENCY BARRIERS (HISTORICAL)

----------------------------------------

[!] This section is marked as HISTORICAL: For more up-to-date information,

including how compiler transformations related to pointer comparisons can

sometimes cause problems, see Documentation/RCU/rcu_dereference.rst.

[!] This section is marked as HISTORICAL: it covers the long-obsolete

smp_read_barrier_depends() macro, the semantics of which are now implicit

in all marked accesses.  For more up-to-date information, including

how compiler transformations can sometimes break address dependencies,

see Documentation/RCU/rcu_dereference.rst.



As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for

DEC Alpha, which means that about the only people who need to pay attention